Caption: Methane. Computer model of 20 molecules of methane (CH4). The atoms (spheres) are colour-coded, for a carbon atom (blue) surrounded by hydrogen atoms (white) in tetrahedral symmetry. Rods are used to show the bonds holding the atoms together to form the molecules. Methane is the simplest alkane. It is a colourless, odourless gas, and is the chief constituent of natural gas, used as a fuel. It is also formed by the decay of vegetable matter. When released into the atmosphere, it helps to trap the heat of the Sun, acting as a greenhouse gas and so contributing to global warming.